The most typical apparatus for mold clamping comprises a lower table and an upper table respectively having molds mounted thereon and thereunder, four guide posts for guiding the two tables to stably move up or down therealong, and lower and upper hydraulic piston-cylinder devices so that when the molds are contacted the piston-cylinder devices are actuated to give additionally increased clamping forces thereto.
However, such piston-cylinder device must be of a few to several tons of power in general and since the machine casing parts must endure such high power, they must be made of a material having high tenacity and rigidity. Forthermore, it takes so much time in order to complete one cycle of operation inclusive of finally exerting additionally increased clamping force.